Entering service one year after the M10, the M18 Hellcat was arguably the truest manifestation of the US's tank destroyer doctrine, using its great speed for hit and run attacks against the much slower German panzers and assault guns. Rather than use the chassis of the M4 Sherman (as was the case with the M10 and M36), the M18 featured a unique chassis (one of the few developed in the US during the war years) with a torsion bar suspension, and powered by a Continental radial engine. However, the M18s top speed of 55 mph was achievable because of the extremely thin armor, which was never more than half an inch at its thickest. Combat service of the M18 began in Italy during the Anzio landings, and it saw extensive use in that campaign as well as in Northwest Europe, notably in the Battle of the Bulge. Although its 76-mm gun was no longer as effective against German tanks like the Panther or the Tiger II, the M18s speed enabled it to outflank enemy tanks and score hits against their weaker side armor. M18s were highly praised by their crews but most were retired after the war, with some being exported to various countries including the former Yugoslavia, the Republic of China, and Venezuela.

A prototype for a new gun motor carriage equipped with a 37-mm gun, twin Buick engines and a Christie suspension was ordered in late 1941, but eventually this was decided to be upgunned with a 57-mm gun as the T49, and later with a 75-mm gun as the T67. It was later decided to adopt the more powerful 76-mm gun M1A1 gun, a torsion bar suspension, and a Continental radial engine; this was then known as the T70, later redesignated M18 76-mm Gun Motor Carriage. No other versions of the basic tank destroyer was built but non-combat variants included the M39 Armored Utility Vehicle which had the turret replaced with an open-topped superstructure. This was used as a command and reconnaissance vehicle in the Korean War, as well as a gun tractor.